Proliferative diseases, such as cancer, are characterised by uncontrolled and unregulated cellular proliferation. Precisely what causes a cell to proliferate in an uncontrolled and unregulated manner has been the focus of intense research over recent decades.
One important class of enzymes that has been the subject of extensive study in this regard is the protein kinase family. The protein kinase family is one of the largest in the human genome, comprising 500 genes. The majority of kinases contain a 250-300 amino acid residue catalytic domain with a conserved core structure. This domain comprises a binding pocket for ATP, whose terminal phosphate group transfers covalently to its macromolecular substrates. The protein kinases may be categorized by the substrates they phosphorylate, e.g. protein-serine/threonine, protein-tyrosine.
Protein kinases mediate intracellular signalling by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signalling pathway. These phosphorylation events are triggered in response to a variety of extracellular and other stimuli and act as molecular on/off switches that can modulate or regulate the target protein biological function. An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, and regulation of the cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include, but are not limited to allergies and asthma, Alzheimer's disease, autoimmune diseases, bone diseases, cancer, cardiovascular diseases, inflammatory diseases, hormone-related diseases, metabolic diseases, neurological and neurodegenerative diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
A wide variety of molecules capable of inhibiting protein kinase function through antagonising ATP binding are known in the art. In particular, it has been disclosed that 2-anilino-4-heteroaryl-pyrimidine compounds (Wang, S.; et al. WO 2003029248, Cyclacel Limited, UK. Fischer, P. M., WO2002079193, Cyclacel Limited, UK. Wang, S.; Fischer, P. M. US2002019404, Cyclacel Limited, UK.; Fischer, P. M.; Wang, S. WO2001072745, Cyclacel Limited, UK), and 2-anilino-4-phenyl-pyrimidine compounds (Wang S., et al. WO2005012262, Cyclacel Limited, UK) possess kinase inhibitory properties, particularly against cyclin-dependent kinases (CDKs).
Cyclin-dependent kinases (CDKs) are serine/threonine protein kinases that associate with various cyclin subunits, playing pivotal roles in the regulation of cell cycle progression and transcriptional cycle. Ten distinct CDKs (CDK1-9 and 11) are involved in a variety of important regulatory pathways in eukaryotic cells, including cell-cycle control, apoptosis, neuronal physiology, differentiation and transcription.
CDKs may be classified into two major groups, reflecting their functions. The cell cycle regulator CDKs composed primarily of CDK1, CDK2, CDK3, CDK4 and CDK6 function with their cyclin partners including cyclin A, B, D1, D2, D3, E, and F to regulate promotion of the cell cycle. The transcription regulator CDKs, which include CDK7, CDK8, CDK9 and CDK11 work together with cyclin C, H, K, L1, L2, T1 and T2, tend to play roles in transcriptional regulation.
The CDKs have been implicated in cell proliferation disorders, particularly in cancer. Cell proliferation is a result of the direct or indirect deregulation of the cell division cycle and the CDKs play a critical role in the regulation of the various phases of this cycle. Therefore, inhibitors of CDKs and their associated cyclins are useful targets for cancer therapy.
CDKs also play a role in apoptosis and T-cell development, which is predominantly due to the CDK functions in regulation of transcription. For example, clear clinical activity has very recently been obtained in chronic lymphocytic leukaemia (CLL) with CDK inhibitor flavopiridol. CLL is characterised by cellular resistance to apoptosis through up-regulation of anti-apoptotic proteins. Inhibition of transcription at the level of CDK9, which is necessary for mRNA elongation, selectively reinstates apoptosis in CLL cells. There is however a need for pharmacologically and pharmaceutically superior CDK inhibitors with a well-defined kinase selectivity and cellular specificity profile and anti-CLL efficacy, as well as efficacy against other CDK mediated disorders.
Furthermore, numerous viruses require CDKs, particular CDK2, CDK7, and CDK9, for their replication process. CDK inhibitors that restrain viral replication including human immunodeficiency virus, human cytomegalovirus, herpes virus, and varicella-zoster virus have been reported.
Inhibition of CDKs, in particular CDK9, is a novel strategy for potential treatment of cardiovascular diseases including cardiohypertrophy. Cardiohypertrophy is characterised by global increases in mRNA and protein synthesis. CDK7 and CDK9 are closely associated with cardiac hypertrophy as they are the main drivers for transcription. Therefore inhibition of CDK9 and its associated cyclins is a relevant drug target for cardiovascular diseases.
Inhibition of CDKs is also useful for the treatment of neurodegenerative disorders such as Alzheimer's disease. The appearance of Paired Helical Filaments, associated with Alzheimer's disease, is caused by the hyperphosphorylation of Tau protein by CDK5/p25.
International Patent Publication No. WO2009/118567 (Cancer Research Technology Limited) discloses a series of substituted-2-anilino-4-arylpyrimidines and substituted-4-aryl-[1,3,5]triazin-2-ylphenyl amines that possess broad therapeutic application as protein kinase inhibitors. The compounds described in WO2009/118567 are all potentially useful therapeutic agents for the treatment of diseases or conditions in which protein kinase hyperactivity (and, in particular, CDK hyperactivity) is implicated.
However, there remains a need to identify new therapeutic agents that can be used to treat such conditions. In particular there is a need to identify further compounds that function as inhibitors of protein kinase (and especially CDK) activity and which also possess one or more advantageous pharmaceutical properties. The one or more advantageous pharmaceutical properties may be selected from the group consisting of: increased potency/target activity (such as increased anti-proliferative activity); increased therapeutic efficacy (such as increased activity against certain cancer cell lines and/or improved selectivity for cancer cells); and/or improved bioavailability (such as oral bioavailability).